Remove legacy Operation code paths

[wjones127]

Parent PRD

Milestone: Action-based Transactions (UserOperation) — see milestone description for full PRD.
Discussion: #5960

What to build

Once all 14 action types are implemented and all 15 Operation variants translate through them, remove the old per-Operation logic:

• Old build_manifest match arms (the per-Operation apply logic)
• Old per-Operation conflict resolution methods (check_delete_txn, check_rewrite_txn, check_create_index_txn, etc.)
• Any dead code from the old path

The Operation enum itself may remain for deserialization of old transaction files, but its apply/conflict logic is fully replaced by the Action path.

Acceptance criteria

• Old per-Operation build_manifest logic removed
• Old per-Operation conflict resolution logic removed
• All existing tests still pass through the Action code path
• No regression in transaction processing behavior

Blocked by

• Blocked by RemoveFragments + UpdateFragment actions (internal) #6450 (RemoveFragments + UpdateFragment)
• Blocked by Schema actions: AddFields, DropFields, UpdateSchemaMetadata (internal) #6451 (Schema actions)
• Blocked by Index actions: AddIndex, RemoveIndex, UpdateMemWalIndex (internal) #6452 (Index actions)
• Blocked by Config/metadata actions: UpdateConfig, UpdateTableMetadata, SetDataFormat, SetFeatureFlags, ReserveFragmentIds (internal) #6453 (Config/metadata actions)

User stories addressed

• User story 5: single code path for applying operations
• User story 6: single code path for conflict resolution